The invention relates to a hydrophobically associating hydroxyethylcellulose oil displacement agent. The hydrophobically associating hydroxyethylcellulose has the constitutional unit shown in the formula I. In the formula I, R is -C12H25, -C14H29 or -C16H33. The hydrophobically associating hydroxyethylcellulose is used to prepare 2000-8000mg/L of solution and the solution is injected in formationor core so as to increase the oil recovery. In the oil displacement agent, the modified natural polymer-hydrophobically associating hydroxyethylcellulose is used to replace the refractory polyacrylamide oil displacement agent with potential toxicity.